1. Field of the Invention
The invention relates to a monolithic substrate preferably made of glass or glass ceramics, which substrate has a lightweight structure, and relates to a method for manufacturing same. Due to its lightweight structure, the weight of the substrate can be significantly reduced, and at the same time a high rigidity can be ensured. The substrate is preferably used as a mirror or a mirror support and can be employed terrestrially and/or extra-terrestrially.
2. Description of Related Art
Large substrates having a low thermal expansion coefficient are frequently employed in astronomy, especially for large mirror telescopes. Such substrates are often used as a mirror support for attaching optical devices such as mirrors, or, after appropriate additional processing steps, are even directly used as a mirror.
The employed substrates have, e.g., circular, elliptical, hexagonal, or even honeycomb-shaped outer contours. The substrates can have diameters of about 1,000 millimeters (mm) or more. From high performance mirror telescopes, mirror supports with diameters of about 6,000 mm or more are even known.
When used as an optical device, it is important that the substrate has a high rigidity and thus only a very low tendency to deformation or alterations in geometry, to prevent unwanted optical effects, e.g. to avoid distortions.
Geometrical alterations may be caused by strong and/or rapid temperature fluctuations which may occur in an extra-terrestrial environment in satellites, or even in a terrestrial environment subjected to great temperature changes between day and night.
For this reason, the substrate often comprises glass or glass ceramics, and, in the case of glass ceramics, can comprise so-called zero-expansion materials which are characterized by an extremely low thermal expansion coefficient. In this manner it is possible to largely eliminate deformations of the substrate or alterations of its geometry caused by temperature changes.
Moreover, relevant alterations in geometry, such as sagging, may even occur due to a heavy proper weight of the substrate. For this reason, a weight reduction of the substrate is often desired.
A reduction of the weight has also to be intended for reasons of a simpler and cheaper transport, especially in case of extra-terrestrial applications. In addition, if the mirror or mirror support has a lower weight, adjustment thereof is simplified since the actuating or readjustment forces are directly related to the mass of the substrate to be moved.
Therefore, substrates have been developed which have so-called lightweight structures. To reduce weight, a part of the volume of the substrate is removed by some treatment. The treatment is often carried out on the rear face of the substrate. This allows appropriate structures to be formed, which can ensure high rigidity.
For example, recesses are known on the rear face of substrates which result in webs of a honeycomb or tubular structure. For example, DE 10 2009 005 400 A1 describes a mirror support with a lightweight structure which has recesses arranged on the rear face of the substrate, and the recesses in the bearing area are provided with a cover, for increased rigidity. Here, the covers may have the shape of the recess and can be adhered onto or into the recess.
It has been found that with the embodiment shown in DE 10 2009 005 400 A1 a high weight reduction can be achieved. However, the production of the covers and connection thereof to the substrate result in additional costs.
Other known lightweight structures are often made of composite material, and additional support components are employed.
For example, WO 2006/034 775 A1 shows a composite structure made of a zero-expansion material, comprising a mirror component and a support component that are coupled by tubular spacers which are arranged in parallel, and an adhesive is used for bonding.
EP 0395 257 A2 shows a composite structure of materials in which a honeycomb-shaped support structure made of sintered ceramic struts is used.
A use of different materials is generally not preferable because different expansion coefficients can result in local stresses in the material and thus can cause damage. Moreover, processing and subsequent joining of several components, such as by an adhesive, imply additional costs. When using an adhesive or glue, attention has to be paid to long-term stability, especially in extra-terrestrial applications.
Therefore, an object of the invention is to provide a lightweight structure for substrates, which on the one hand eliminates incorporation of additional members or support components such as spacers, struts, covers, or the like, and on the other hand exhibits an approximately equivalent mechanical stability to dynamic and static stresses.